What if BP’s relief wells don’t plug the leak?, Reuters, July 9, 2010:

“I think this situation has taught us from the start to have a backup,” retired U.S. Coast Guard Admiral Thad Allen, the top official overseeing the spill response, told reporters this week. …

If both relief wells fail, Allen said BP has a plan to fabricate a new pipeline, place it on the seabed and hook it up to the leaking wellhead. …

[Donald Van Nieuwenhuise, director of petroleum geoscience programs at the University of Houston] said BP also could possibly detonate a bomb — conventional or nuclear — in the well to try to stop the flow.

But a blast could damage seabed oil and gas pipelines or cause an undersea mudslide…

“The big danger with any kind of implosion method is that it’s a wild card, and you don’t know what other kinds of problems it would bring,” Nieuwenhuise said.
Undersea Landslide to Blame for Deadly Wave; Scientists: Conditions for Tsunami May Exist in U.S., ABC News, July 18, 2000:

[D]iscoveries are drawing our attention to other causes of tsunamis, besides the traditional tectonic earthquake, [Eddie Bernard, director of the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory] said. The more we learn about possible causes, the better we can know when to issue warnings.

Offshore areas most prone to landslides are those where eons of runoff sediments from rivers have created terraces built of loose materials, he said.

When it breaks loose, the material drops with the speed of a snow avalanche, displacing the water below and leaving a void the water fills with a bump that spawns the localized tsunami, Bernard said. …

Tsunamis move at 500 to 600 mph in deep ocean waters but slow and get taller as they reach shallow offshore waters.
The Gulf of Mexico is rarely affected by earthquakes. Any slight movement is likely to displace loose sediments that have not moved in a very long of time.

BP’s blown-out well sits in the path of the Mississippi River Delta, around 50 miles offshore. Louisiana State University geologists estimate between “2.79 trillion and 3.45 trillion tons of sediment have been stored in the delta since the end of the last glacial maximum.”